Just take a peak at the mobo. I think (If im wrong, well this is like my fourth post) and the small colored things (Follow the colored wire from you HDD if you cant find them) should be SATA connecters. How ever many of those you have is how many SATA HDDs that mobo can handle.

I think how many drives your pc can support is mostly dependent on your mobo like someone recently stated, but if you want a lot of sata ports, go with one of the new server mobo's with sata arrangements.